Phase instability is a common problem associated with swept source optical coherence tomography (SS-OCT) systems. There are many potential causes of SS-OCT phase instability including asynchronization between the light source (e.g., a sweeping laser) in the OCT system and the electrical sweep trigger (for synchronizing the beginning of each tuning period of the light source), and trigger uncertainty inherent in the analog-to-digital conversion process. Additionally, trigger fluctuation may occur when the A-scan trigger is not synced to a specific wavelength. In this case, signal acquisition is not triggered at the same wavelength for each A-scan. Light source sweeping variation may occur when the light source does not sweep through the same optical wavelength range repeatedly every time. In those cases, the acquired signals will not have the same sampled signal. Clock asynchronizations (e.g., between an optical clock, A-scan trigger, and light source of an SS-OCT system) can create a shift in sampled signals when the clock is not perfectly synchronized to the A-scan trigger. Signal shifts may also result from data acquisition instability and timing delays.
A number of existing approaches have thus attempted to solve phase stabilization problems using a reference signal(s) acquired by a second channel or optically mixed with imaging signals. Some of these methods incorporate new elements in the OCT system or require an optical clock. One approach includes a calibration mirror corresponding to a tissue depth, so as to generate a calibration signal associated with that depth. Other methods introduce a wavelength signature signal with a fiber Bragg grating or a gas cell. Still other methods use cross-correlated corrected phase of a Mach-Zehnder Interferometer (MZI) to stabilize A-scans. Still further methods consider phase differences to generate negative cancelling signals.
Separately, multimodal OCT refers to an OCT system that has adjustable OCT imaging parameters such as imaging speed, imaging range, and imaging resolution. Parameters such as sensitivity, sampling rate, and optical wavelength range are also tied to imaging speed, imaging range, and imaging resolution, respectively. Thus, tradeoffs in these parameters exist in order to maximize performance of the OCT system. For example, with a fixed sampling rate, decreasing imaging speed (sensitivity) or imaging resolution (optical wavelength range) increases imaging range. In these multimodal OCT systems, the clock may be an optical clock including two different MZIs having different properties, selectable by a controller depending a desired operation. As a result, images taken in different modes of multimodal systems can also exhibit sampling differences between A-scans.